1. Field of the Invention
The present invention relates generally to a seek control method in an optical storage device, of seek-moving a light beam to a target track on an optical storage medium and, more particularly, to a seek control method in an optical storage device using a digital signal processor.
2. Description of the Related Art
An optical storage device such as an optical disk device, an optical card device, etc. has been widely utilized as a storage device. In this kind of optical storage device, an optical storage medium is irradiated with light beams, and data are thus written to and read from the optical storage medium.
In this optical storage device, a so-called seek operation of moving the light beam to a desired track is conducted. In this seek operation, a track error signal is generated by the light beam reflected from the optical storage medium in order to detect a position of the light beam. When detecting a present position from such a track error signal, even though digital servo control is carried out, a technology for precisely detecting the present position is desired. Further, a technology for performing the servo control with no delay is also desired.
As a conventional method of detecting the present position from a track error signal TES, there has been known a method of detecting the present position from an analog value of the track error signal.
Further, according to another known method, a track zero-cross signal TZC is obtained by slicing the track error signal at a zero level, and the present position is detected by counting those track zero-cross signals.
However, a frequency of the track error signal TES during the seek is as high as 500 kHz at a maximum velocity. For this reason, it may happen that a one-sample process extends 10 or more tracks at a sampling frequency of 50 kHz-40 kHz of the digital servo. Further, the track error signal TES during the seek has a smaller amplitude with an increase in moving velocity.
Therefore, according to the method of detecting the present position from the analog value of the track error signal, if a seek velocity is high, there arises a problem wherein it is difficult to detect the present position with an accuracy on the order of one or less track.
Moreover, according to a method of calculating the present position from the zero-cross signal, if the seek velocity is high, the present position can be stably detected. If the seek velocity is low, however, a problem is that an exact position is hard to detect because of the velocity being unstable.
Next, according to a feedforward control method, feedforward quantities in accelerating and decelerating directions are applied to an acceleration period and a deceleration period as well. In a sample servo control system using a processor for a servo control circuit, the present position is detected from the track error signal at each sample timing. Then, after calculating a target position at that sample timing, a target velocity and a real velocity are to be calculated.
Then, an error quantity between the target velocity and the real velocity is calculated. Furthermore, there is decided whether or not the detected present position is at a predetermined start-of-acceleration point or start-of-deceleration point. Then, the detected present position coionsides with one of the predetermined start-of-point, the feedforward quantity in the acceleration or deceleration direction is generated. This feedforward quantity is added to the above error quantity, thus obtaining a control quantity. Based on this control quantity, a motor for an optical head moving mechanism is driven.
The motor for this head moving mechanism actualizes a high-velocity seek and therefore requires a large torque. For this reason, a coil of the motor has a large inductance component. Consequently, an actual current of the motor is late to start flowing. Therefore, the real velocity has a time-lag deviating from a target velocity curve.
Further, in the above-mentioned sample servo system, the time-lag is produced in the target velocity curve during a period from a timing of giving a start-of-acceleration indication or a start-of-deceleration indication to a time when the feedforward quantity is outputted.
Hence, at the time when a feedforward quantity is outputted, the control is performed to compensate a delay of an actual start of the deceleration, resulting in an increased error quantity. That is, an overshoot quantity augments. The servo system is thereby made unstable. Therefore, according to the prior art, it is difficult to locate the light beam to the target track.
Further, the same problems may also arise during the acceleration. Therefore, the control is effected to compensate a delay of an actual start of the acceleration, and hence the error quantity augments, resulting in an increased overshoot quantity. Hence, according to the prior art, constant velocity control is hard to perform.
Moreover, if the above-mentioned overshoot quantity increases, noises of a motor also become louder.